A Charge Coupled Device (CCD) is a series of Metal Oxide Semiconductor (MOS) capacitors closely located next to each other so that signal charge can be transferred from one capacitor to the next in shift register like action. For example, CCDs may be used in image sensors to transfer photo-generated signal charges from light sensitive capacitors to an output.
FIG. 1 illustrates a single-poly, surface-channel CCD shift register formed in a MOS or a Complementary Metal Oxide Semiconductor (CMOS) process. FIG. 1A shows a cross-section of an example of a CCD shift register, and FIG. 1B shows a plan view of the CCD shift register. The example CCD shift register includes three gates, 1, 3 and 8, separated by gaps 2 and 7. The gates reside above a gate-oxide layer 4, over a region of active semiconductor 5. The active region is surrounded by an isolation region 9. The gap regions are filled with a dielectric material. A charge packet 6 is held in an inversion layer under gate 1, at the top surface of the semiconductor (adjacent to the gate oxide), and can be transferred to the adjacent and subsequent gates. When charge is transferred from gate 1 to gate 3, for example, it may pass along the semiconductor surface underlying the gap region 2. A typical CCD shift register has many gates similar to the three shown in FIG. 1.